The invention relates to a method for controlling the flow of a fluid through high permeability strata, fractures and high permeability channels (wormholes), in a subterranean formation. An example is controlling the influx of water during the recovery of hydrocarbons from geological formations. In particular, the invention relates to chemical gel systems containing a reinforcing agent, (e.g., produced sands) for permeability modification of such formations.
During the recovery of hydrocarbons from subterranean formations, significant amounts of hydrocarbons are left behind because injected or natural drive fluids in the formation are produced along with the oil to such an extent that the cost of fluid disposal makes further oil recovery uneconomical. In formations with high permeability strata, fractures, or high permeability channels (wormholes), natural drive fluids (such as brine or gaseous hydrocarbons) in primary recovery processes or flooding fluids (such as brine, steam or carbon dioxide) in secondary recovery processes flow through highly permeable zones, resulting in progressively less hydrocarbon being recovered per unit volume of fluid produced. This increased ratio of drive or flooding fluid to hydrocarbons is usually due either to early breakthrough of flooding fluid from injector wells to producer wells, or to excessive water encroachment into producer wells. It has adversely affected the economics of recovery processes in many parts of the world. For example, it was recently estimated that in the United States, 7 barrels of water are produced for each barrel of oil, amounting to 2.1xc3x971010 barrels of water annually. In Alberta, Canada, the ratio of water/oil produced is 6/1, amounting to 3.0xc3x97109 barrels of water produced in 1997.
The control of fluid flow in subterranean formations is commonly referred to as xe2x80x9cconformance controlxe2x80x9d. For the past two decades, research has been directed at improving the oil/water ratio during hydrocarbon production by using chemical gel systems to block water flow through high permeability zones, fractures and high permeability channels (referred to herein as xe2x80x9chigh permeability regionsxe2x80x9d). The general approach has been to inject a mixture of reagents, initially low in viscosity, into regions of a formation which have high permeability. Once the mixture of reagents has reached its destination in the desired region of the formation, it then undergoes a chemical reaction to produce a gel which is capable of blocking the flow of water. Polymers, chemical gels, silica gels, and other blocking agents have been used in this way for conformance control in geological formations.
Ideally, a gel system for conformance control should have the following properties:
1. The reagents should be easily delivered to the desired location in the formation. The components therefore should be initially of low viscosity. No component should be adsorbed out prior to reaching its destination, and each component should be stable to shear stress encountered during delivery.
2. The chemical reaction(s) required for gelation under the conditions found in the formation.
3. The gel generated should be of high strength under the conditions found in the formation or its strength reinforced with a readily accessible, low cost reinforcing agent.
4. The degree of permeability reduction should be high.
5. The system should be of low enough cost to make it economically feasible.
6. The system should have minimal environmental impact.
All of the chemical gel systems currently available for conformance control have the drawback of being so costly that their use is limited. Examples of existing gel systems are:
1. Polyacrylamide copolymers which are injected together with a cross-linking agent, and a chromium (III) or aluminum (III) compound;
2. Xanthan gum (a natural heteropolysaccharide) which together with a cross-linking agent, and a chromium (III) compound;
3. Poly (Vinyl alcohol) which is injected together with a cross-linking agent, and gluteraldehyde; and
4. Acidified sodium silicate, which when neutralized, rapidly undergoes polymerization to form spherical silica particles.
The most widely used method of these involves use of polyacrylamide cross-linked with chromium ions. Its use is limited by its cost: 1 m3 polyacrylamide costs about 200 to 500 USD, while typical applications use from about 20 m3 to about 300 m3. It is also relatively unstable under the elevated temperature conditions which exist in deep geological formations or reservoirs undergoing thermal recovery processes. Furthermore, chromium (VI), the oxidation product of chromium (III) is highly toxic, so the use of a chromium (III) compound as a cross-linking agent can be an environmental concern.
Another application for a conformance control gel system is to block permeable regions to contain a fluid within a certain region. This may be particularly applicable to the disposal of fluids in a subterranean formation, i.e. to reduce the flow of a disposal fluid into other regions.
There is therefore a need to develop further conformance control gel systems which are environmentally safe, inexpensive and effective under the conditions encountered during hydrocarbon recovery.
In accordance with the present invention, a method is provided for method for controlling the flow of at least one fluid in a subterranean formation having at least a first region, said first region having (i) at least Na+, and (ii) a first permeability, K1, with respect to said fluid, said method comprising: (a) making an inhibitive electrolyte solution having water and at least one inhibitive compound, said inhibitive compound having at least one cation and anion; (b) making a clay/reinforcing agent slurry by mixing at least about 15 weight percent of a swelling clay with said inhibitive electrolyte solution so that, to the extent clay gel is produced, if any, the flowability of said slurry is not substantially inhibited and mixing a reinforcing agent in the range of from about 10 weight percent to about 60 weight percent; (c) injecting said clay/reinforcing agent slurry into said formation, so that at least a portion of said slurry contacts said first region; (d) allowing a reinforced clay gel to form in said first region so that K1, is reduced to produce a lower permeability, K1L, with respect to said fluid; and (e) controlling the flow of at least a majority of said at least one fluid into or from said first region.
In accordance with the present invention, there is also provided method for controlling the flow of at least one fluid in a subterranean formation having at least a first region, said first region having (i) at least Na+, and (ii) a first permeability, K1with respect to said fluid said method comprising: (a) making a first inhibitive electrolyte solution having water and at least one inhibitive compound, said inhibitive compound having at least one cation and anion; (b) treating a swelling clay with said first inhibitive electrolyte solution; (c) making a clay/reinforcing agent slurry having at least about 15 weight percent of the treated clay of step (b) and a second inhibitive electrolyte solution having water and at least one inhibitive compound and mixing a reinforcing agent in the range of from about 10 weight percent to about 60 weight percent; (d) injecting said clay/reinforcing agent slurry into said formation, so that at least a portion of said slurry contacts said first region; (e) allowing a reinforced clay gel to form in said first region so that K1, is reduced to produce a lower permeability, K1L, with respect to said fluid; and (f) controlling the flow of at least a majority of said at least one fluid into or from at least said first region.